You can find more information in the stan manual: http://mc-stan.org/users/documentation/. See part 47. In brief it allows you to put down the system of equation in your model as constrain without specifically work out how to express the solution.
You can find more information in the stan manual: http://mc-stan.org/users/documentation/. See part 47. In brief it allows you to put down the system of equation in your model as constrain without specifically work out how to express the solution.